The original speculated resolution of 2304 × 1440 made little sense. What resolution would a 12” Retina MacBook Air feature instead? The 11.6” MacBook Air currently comes with a 1366 × 768 panel. Pixel quadrupling this resolution would represent the easiest path towards a Retina MacBook Air in terms of meeting performance and power constraints, yielding a 2732 × 1536 display.
What size would this panel be? Assuming that the rumor of an approximately 12” Retina MacBook Air has some veracity, why not something close to it?
It turns out that an ~11.88” Retina MacBook Air with a 2732 × 1536 resolution happens to have the exact same pixel density as the 9.7” 2048 × 1536 Retina iPads: ~264 PPI. It would make sense for Apple to take advantage of the same display technology it has been utilizing for the 9.7” iPads by cutting their panels to this larger size. I suspect a new 11.88” Air would be a redesign of the 11.6” model with smaller screen bezels.
The difference is, with the iPads and iPhones, a straight doubling the number of pixels across the same physical space had benefits beyond than manufacturing efficiency. For iOS, a pixel-doubled display meant better app compatibility. Pre-existing apps would just run in a 2x mode and developers only had to supply new assets to take advantage of the ‘Retina’ fidelity.
If Apple had chosen any other resolution for the iPhone 4 or iPad 3, developers would have had to redesign their apps from scratch and compatibility mode would have comprised horizontal and vertical letterboxing.
For OS X, unlike iOS, pixel-doubling is not a requirement. Due to the windowed nature of OS X, apps can run on devices with a whole variety of screen resolutions. There may be cost savings for Apple (by using iPad display sheets at a different size), but I can’t imagine that the savings are so significant as to dictate Apple’s course.1
Ignoring the above issues, I still think extrapolating the Air’s display from the iPad’s display is a baseless argument. Apple could just as easily use the 15 inch MacBook Pro’s display (which has a DPI of 216), or the 13 inch MacBook Pro’s display (which has a DPI of 226), or something else entirely. Each of those options produce different end results for an ~11inch Air. The point is, you can’t tell.
1 You save money on the display components, but you still have to pay for tooling, like bigger production lines.